Electronic circuits can be susceptible to damage or operational failure as a result of Electrostatic Discharge (ESD) events or Electrical Over-stress (EOS) events that occur on their terminals. An ESD event or an EOS event may occur, for example, when a user who has accumulated electrostatic charge picks up the integrated circuit, touches an exposed pin, or, for example, interacts with a touch pad coupled to the integrated circuit. As used herein, the term “ESD event” refers to a transient surge of energy, which may be as high as a few thousand volts, that appears across the terminals of the integrated circuit. As used herein, the term “EOS event” refers to a rapid change in energy that appears on terminals of the integrated circuit, which change may be greater than an application-specific threshold and which can cause an observable change in the behavior of the circuit. While EOS events can be produced through user interactions with the circuit, EOS events can also be created within the chip itself, such as through switching activity of a large input/output driver circuit. Regardless of the source, such EOS events may disrupt operation of the circuit (such as by corrupting measurement data) or may cause long term damage by over-stressing sensitive circuitry.
Most integrated circuits incorporate ESD protection structures that are coupled to input/output (I/O) pins and to the power supply terminals to protect other circuit structures on the integrated circuit. Such ESD protection structures are designed to activate in response to a transient voltage in excess of a pre-determined energy threshold, which is usually at a voltage level that is below a voltage rating for associated circuitry. In a particular example, if a logic circuit of the integrated circuit is rated to withstand voltages up to approximately 7.0 volts, the ESD energy threshold would be set at a level below 7.0 volts, so that the ESD protection structure is activated to clamp the voltage before the voltage rises to a level that would damage the other circuitry. It should be understood that the above numbers are illustrative only, and that other voltages, both lower and significantly higher, may be used as thresholds in such circuits. In many instances, such ESD protection structures operate to clamp the input voltage at a pre-configured voltage level and to divert excess energy from the ESD event to one of the power supply terminals in order to prevent damage to the circuitry from such high-energy transients.
EOS discharges below the ESD threshold level may not activate the ESD protection structure. However, the elevated voltage and/or current peaks associated with these discharges can nevertheless disrupt circuit operation and may damage associated circuitry. Circuit functions that involve the generation or measurement of very small voltages or currents, such as analog-to-digital converters, are particularly susceptible to corruption from such EOS events. Such corruption can include short-term disruption of measurement data and/or permanent damage to measurement circuitry due to over-stress. Furthermore, it can be very difficult to determine when such corruption occurs, since the EOS-induced perturbation in the integrated circuit's output may be similar to one caused by a valid change to an analog input signal.